package com.net.common.util;



import java.util.Date;

import static com.net.common.util.DateUtils.YYYY_MM_DD_HH_MM_SS;

public class SnowflakeIdGenerator {

    /**
     * 开始时间截 (2000-01-01)
     */
    private final long twepoch = 946656000000L;

    /**
     * 机器id所占的位数
     */
    private final long workerIdBits = 5L;

    /**
     * 数据标识id所占的位数
     */
    private final long dataCenterIdBits = 5L;

    /**
     * 支持的最大机器id，结果是31 (这个移位算法可以很快的计算出几位二进制数所能表示的最大十进制数)
     */
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);

    /**
     * 支持的最大数据标识id，结果是31
     */
    private final long maxDataCenterId = -1L ^ (-1L << dataCenterIdBits);

    /**
     * 序列在id中占的位数
     */
    private final long sequenceBits = 12L;

    /**
     * 机器ID向左移12位
     */
    private final long workerIdShift = sequenceBits;

    /**
     * 数据标识id向左移17位(12+5)
     */
    private final long dataCenterIdShift = sequenceBits + workerIdBits;

    /**
     * 时间截向左移22位(5+5+12)
     */
    private final long timestampLeftShift = sequenceBits + workerIdBits + dataCenterIdBits;

    /**
     * 生成序列的掩码，这里为4095 (0b111111111111=0xfff=4095)
     */
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    /**
     * 工作机器ID(0~31)
     */
    private long workerId;

    /**
     * 数据中心ID(0~31)
     */
    private long dataCenterId;

    /**
     * 备用的数据中心ID(0~31),当时钟回拨时，为了不抛异常，启用备用ID
     */
    private long standByDataCenterId;

    /**
     * 毫秒内序列(0~4095)
     */
    private long sequence = 0L;

    /**
     * 上次生成ID的时间截
     */
    private long lastTimestamp = -1L;

    /**
     * 创建实例,这里不同项目机器或项目需要设置不同workerId和dataCenterId
     */
    private final static SnowflakeIdGenerator ID_GEN_INSTANCE = new SnowflakeIdGenerator(0, 0, 1);

    /**
     * 构造函数
     *
     * @param workerId     工作ID (0~31)
     * @param dataCenterId 数据中心ID (0~31)
     */
    private SnowflakeIdGenerator(long workerId, long dataCenterId, long standByDataCenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (dataCenterId > maxDataCenterId || dataCenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDataCenterId));
        }
        if (standByDataCenterId > maxDataCenterId || standByDataCenterId < 0) {
            throw new IllegalArgumentException(String.format("standby datacenter Id can't be greater than %d or less than 0", maxDataCenterId));
        }
        if (dataCenterId == standByDataCenterId) {
            throw new IllegalArgumentException(String.format("datacenter Id %s can't equal to standby datacenter Id %s.", dataCenterId, standByDataCenterId));
        }
        this.workerId = workerId;
        this.dataCenterId = dataCenterId;
        this.standByDataCenterId = standByDataCenterId;
    }


    /**
     * 获得下一个ID (该方法是线程安全的)
     *
     * @return SnowflakeId
     */
    private synchronized long nextId() {
        long timestamp = timeGen();
        if (timestamp < twepoch) {
            throw new RuntimeException(
                    String.format("服务器时间不能小于%s", DateUtils.parseDateToStr(YYYY_MM_DD_HH_MM_SS, new Date(twepoch))));
        }

        boolean isTimestampBack;
        if (timestamp < lastTimestamp) {
            isTimestampBack = true;
        } else {
            isTimestampBack = false;
        }

        //如果是同一时间生成的，则进行毫秒内序列
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            //毫秒内序列溢出
            if (sequence == 0) {
                //阻塞到下一个毫秒,获得新的时间戳
                timestamp = tilNextMillis(lastTimestamp);
            }
        }
        //时间戳改变，毫秒内序列重置
        else {
            //利用时间戳解决id取值平均分配问题
            sequence = 127 & timestamp;
        }

        //上次生成ID的时间截
        lastTimestamp = timestamp;
        long dataCenterIdUse = isTimestampBack ? standByDataCenterId : dataCenterId;

        //移位并通过或运算拼到一起组成64位的ID
        return ((timestamp - twepoch) << timestampLeftShift)
                | (dataCenterIdUse << dataCenterIdShift)
                | (workerId << workerIdShift)
                | sequence;
    }

    /**
     * 阻塞到下一个毫秒，直到获得新的时间戳
     *
     * @param lastTimestamp 上次生成ID的时间截
     * @return 当前时间戳
     */
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    /**
     * 返回以毫秒为单位的当前时间
     *
     * @return 当前时间(毫秒)
     */
    private long timeGen() {
        return System.currentTimeMillis();
    }


    /**
     * 生成唯一ID
     *
     * @return
     */
    public static long getId() {
        return ID_GEN_INSTANCE.nextId();
    }

}
